Analysis of integrated UWB MIMO and CR antenna system using transmission line model with functional verification

This paper discusses the equivalent circuit model and functional verification of an integrated antenna system as its main focus. The integrated antenna system consists of two independent antenna systems, namely the Cognitive Radio antenna and the Ultra Wide Band Multiple Iinput Multiple Output antenna. This article is split into two parts: The first part discusses the equivalent circuit of an integrated antenna system by optimizing the RLC values. The developed lumped equivalent circuit model produces the NB resonant frequencies, which is the same as the S-parameter obtained through EM simulation. The second part of this paper aims to discuss the experimental verification of an integrated CR antenna system with Bayesian learning-based spectrum sensing algorithms using Universal Software Radio Peripheral devices. Real-time sensing and communication functionalities are visualized in the LABVIEW monitor. The integrated antenna system is fabricated and measured after the simulation.


Scientific Reports
| (2022) 12:14128 | https://doi.org/10.1038/s41598-022-17550-z www.nature.com/scientificreports/ Radio (CR) antenna [10][11][12][13][14][15] , integrated CR antennas [16][17][18][19] , dual polarized metasurfraces 20,21 and equivalent circuit analysis of UWB/NB antenna [22][23][24][25][26] mentioned in the literature. Reconfigurable antennas, on the other hand, have the following drawbacks: The biasing circuit is difficult to build, the toggling is influenced by in homogeneity, and the PIN diode needs extra power. The integrated antenna system 27 provides solution for the above mentioned problems. There are two independent antennas in the proposed integrated antenna systems 27 . For the transmission line model analysis and functional verification we have considered our designed antenna 27 . First, part of the antenna system includes the CR antenna(UWB and NB) and second, part includes the UWB MIMO antenna 27 . The UWB antenna for detection and NB antennas to execute communications. The integrated CR and MIMO antennas enhance spectrum utilization and diversity performance because of their dual-polarization. For both sending and receiving, the UWB MIMO antenna radiators are constantly active, but sensing UWB radiators are always active 27 . In addition, the narrowband antennas are idle, but if the UWB antenna detects white space, the associated NB antenna is active for communication 27 . This article introduces the equivalent circuit model of integrated antenna system. Motivation, and contribution. As of now, many of the equivalent circuit models have been implemented for patch antenna, stacked patch antenna and notch filter. Only a few researchers have implemented equivalent circuit models for UWB antennas. In 28 , 29 the transmission model is implemented for simple and stacked general patch antenna was carried out and equivalent circuit of the planar CR antenna system was carried out in 22 , but this article discuss the equivalent circuit model of the entire antenna system 27 . Also, the novel contribution of this work is this is the first article discussing the transmission line equivalent circuit model of entire integrated antenna system (Narrow band antenna, notch filter and UWB antenna) and transmission line equivalent circuit analysis done for the narrow band antennas, notch filter, and UWB antenna.
The main contributions of the proposed hardware implementation is as follows.
• First, we have designed an integrated CR and UWB MIMO antenna 27 .
• The UWB signal is transmitted by the transmitting antenna and received by the UWB sensing antenna (CR antenna), where the sending and receiving antennas are connected to two different USRP's. • From the received UWB spectrum, the white spaces are identified using Bayesian learning-based wideband spectrum sensing algorithms in MATLAB software. • The proposed system hardware implementation is effective because it can be easily implemented with less hardware. Also, it has effective spectrum sensing and white space detection. • Also, the MIMO antenna real time performance is analyzed using two element UWB MIMO antenna.

Equivalent circuit model of integrated(Hybrid) antenna system
The UWB radiator with and without band notch, and narrow band antenna design is carried out in 27 . As a continuation of 27 in this paper we have carried out detailed lumped circuit model analysis of each independent antenna system and real time implementation of the integrated antenna system. Equivalent circuit of narrow band(NB) antennas. This section describes the lumped equivalent circuit model of NB antennas. In general, an antenna is a combination of linear and passive elements network. The antenna's overall equivalent model can be obtained using lumped elements such as resistance (R), capacitance (C), inductance (L) and conductance (G). Any conducting element of the antenna is considered to be a combination of R and L in series as it provides resistance and inductance in a reason of conductivity. Besides, the conductor losses are denoted by R. It is assumed that the capacitor (C) and the conductance (G) are connected in parallel in the antenna because a dielectric material separates the upper and lower conductive layers. Further, when a conductive material two conductors, some capacitance occurs, and G is taken into account due to the dielectric loss. In addition, in order to consider fringe fields and plane waves, a small amount of inductance is taken into account. By considering this, inductance can improve the accuracy of the approximate lumped equivalent model. Initially lumped equivalent model of the fundamental patches (circular, rectangular, square, and other shapes) antenna is presented. However, it is only used for patches that have no slot. The same procedure is followed for the proposed NB antennas. In general, a patch antenna has three conducting elements such as feed line, radiating patch, and the ground plane. These three conducting layers are approximated by three series R and L combinations since each conducting layer form one series R and L combinations. Figure 1a shows the feed line, ground plane, and radiating patch equivalent circuit model represented by R f , L f , R g , L g , and R r , L r , respectively. In addition, the equivalent circuit model between the ground and feed is represented by the parallel combination of C g and G g . Similarly, the parallel combination of C r and G r represents the equivalent circuit model between the ground and the radiating patch. The 50 matched terminator is used to terminate the input port, while the radiating patch is closed with a 377 free space inherent impedance as shown in Fig. 1a. Equivalent model is simplified by using two assumptions, (i) The current flowing through the ground plane is very small compared to the current flowing through the radiating patch, so the series combination of R g and L g of the ground plane is ignored. (ii) Shunt combination of capacitance C g and conductance G g formed between the feed line and ground plane, they are ignored because they contribute very little C and G. Due to this, the R(R f +R r ) and L(L f +L r ) of the feed line and patch is connected in series, and both are added. Eventually, the simplified equivalent circuit model is shown in Fig. 1b,c; it is obtained after considering two assumptions. Based on the second assumption, the resistance R f , R r and L f , L r is represented by (R f +R r ) and (L f +L r ), as shown in the Fig. 1c. Figure 2a-d shows the four /4 length NB monopole antennas. It has a feedline, radiating stub, and ground plane, which is responsible for generating the upper, middle, and lower resonance bands. As discussed for generalized microstrip antenna, the equivalent circuit model designed and simulated for the proposed NB antenna. R r , L r represents the 3mm width radiating stub, and it is connected in series. Furthermore, G 1 and C 1 represent the equivalent circuit model between feedline and ground plane, and they are connected in parallel. Likewise, G 2 and C 2 represent the equivalent circuit model between radiating stub and ground plane, that is connected in parallel. Due to the surface wave and edge field effects, some inductance is created, which is represented by the inductors L 1 and L 2 . The optimized R, L, C element values presented in Table 1. The final lumped element circuit of the four proposed NB antennas are presented in Fig. 3 ; it can be observed from the Fig. 3 the lumped element circuit of all four NB antennas are identical, but R, L, C, and G value of each NB antennas different as mentioned in Table 1. Consequently, S11 of the equivalent lumped element circuit resembles the EM model S11 at its operating frequency. The S-parameter of the lumped element circuit for four NB antennas shown in the

Equivalent circuit model of UWB antenna
This section describes the lumped equivalent circuit model of UWB antenna. The designed UWB antenna is shown in Fig. 2e. The equivalent circuit model for the UWB antenna is challenging; very few researchers have implemented the equivalent circuit model and found the S parameters. This paper has implemented the equivalent circuit model for the proposed UWB antenna by considering the antenna as a two port network as implemented in 23 . Due to numerous adjacent resonant circuits (represented by parallel RLC circuits), UWB antennas can be modeled according to input impedance characteristics. Also, in this method, we have considered approximately 90radiation resistance as a second port, and to represent the distributed elements of the antenna, the transmission lines are incorporated into the equivalent circuit model. In order to match the EM model S-parameter, the equivalent circuit model components values are obtained after so many manual optimization processes. After optimization the final equivalent circuit model for the proposed UWB antenna is shown in the Fig. 5. The proposed UWB antenna equivalent circuit model was optimized and simulated in the Advanced Design System (ADS) software. The transmission line is included in the equivalent circuit to represent the distributed elements of the antenna. Figure 6 Shows the comparison of S11 between the equivalent circuit model and the CST EM model. It can be seen from Fig. 6 that the two S-parameters for both cases having good agreement.

Equivalent circuit of half wave length band notch resonator
The designed UWB antenna with band notch filter is shown in Fig. 2f. In order to study the principles behind the equivalent circuit model of the half-wavelength notch-band resonator, it is derived from the antenna input impedance, which is obtained from the CST EM solver. Further, in order to achieve band notch characteristics, the antenna should be high impedance mismatch at the notch band. From the simulation or measurement result (1)     Table 2.  www.nature.com/scientificreports/ It can be seen from the EM model and the lumped circuit model S-parameters are almost the same. Also, it is witnessed that at the notch band the S11 is greater than -2dB for both EM model and equivalent circuit model due to impedance mismatch. Equation (6) used to achieve band notch at WLAN band. Hence, it is clear from the reflection coefficient the WLAN band notch is effectively achieved by using an L-shaped stub.

Equivalent circuit of folded L-shaped stub notch resonators (ITU-band notch).
For the ITU frequency band, at 8 GHz, the real impedance is 200 . Also, the imaginary value of the impedance varies from +ve to −ve value at 8 GHz. The equivalent circuit for the ITU band notch resonator consists of a series and parallel combination RLC components. The lumped equivalent circuit model of ITU (8 GHz) band notch filter is shown Fig. 8. The optimized lumped elements (R, L, C) values shown in Table 2. It can be seen from Fig. 9 the EM model and the lumped circuit model S-parameters are almost the same. Also, it is witnessed that at the notch band the S11 is greater than − 2 dB for both EM model and equivalent circuit model due to impedance mismatch. Equation (7) used to achieve band notch at WLAN band. Hence, it is clear from the reflection coefficient the ITU band notch is effectively achieved by using a two L-shaped slot.      Operational configuration 3. If the UWB sensing radiators identify white space at 5.5, and 12.8 GHz the NB-A3 will be activated for CR communication purposes during this time other NB antennas are being inactive.
Operational configuration 4. If the UWB sensing radiators identify white space at 5, and 12 GHz the NB-A4 will be activated for CR communication purposes during this time other NB antennas are being inactive.

Simulation and measurement results analysis of the integrated antenna system
After equivalent circuit analysis and EM model analysis, the proposed antenna is fabricated and tested in Vector Network Analyzer(VNA) and anechoic chamber to verify the performance of the antennas as depicted in Fig. 11. Figures 12, 13 and 14 compares the simulated and measured antenna parameters of the integrated antenna system. Initially, Fig. 12 shows the simulated and measured reflection coefficients of the vertically and horizontally polarized UWB sensing radiators and it can be witnessed from Fig. 12 it has 3-13 GHz bandwidth with less than − 10 dB. Secondly, Fig. 14 shows the simulated and measured reflection coefficients of the NB communication antenna and it can be witnessed from Fig. 14 it has ten resonant frequencies (3.3, 7.8, 11.8, 3.5, 8.3, 12.3, 5.5, 12.5, 5, and 12 GHz) over 3-13 GHz with less than − 10 dB. Thirdly, Fig. 13 shows the simulated and measured reflection coefficients of the vertically and horizontally polarized two elements UWB MIMO antenna and it can be witnessed from Fig. 13 it has 3-13 GHz bandwidth with less than − 10 dB except for notch bands(5 and 8 GHz).
Further, Figs. 15 and 16 shows the simulated and measured mutual coupling of the eight-port integrated antenna and it can be witnessed from Figs. 15 and 16 it has less than − 20 dB mutual coupling over 3-13 GHz.
The radiation pattern(simulated and measured) of the UWB sensing radiator, UWB MIMO radiator, and NB radiators are illustrated in Figs. 17a-d and 18a-j. It can be witnessed from the Fig. 17a-d the UWB sensing antenna and UWB MIMO antenna radiator have omnidirectional radiation at 3.5, 6.5, 9, and 10 GHz . Similarly, as witnessed from the Fig. 18a-j the NB antenna has an almost omnidirectional radiation pattern at all NB resonant frequencies (3.3, 7.8, 11.8, 3.5, 8.3, 12.6, 5.5, 12.5, 5, and 12 GHz). www.nature.com/scientificreports/ Further, the gain is varying from 2-75 to 3-7.6 dB but for the notch band, it is less than − 3.4 dB, for sensing radiator and two elements MIMO antenna radiator respectively as shown in Fig. 19a,b. Likewise, the efficiency is varying from 70-80%, and 71-85% except for the notch band but at the notch band, it is less than 45%, for sensing radiator and two elements MIMO antenna radiator respectively as shown in Fig. 20a,b. Finally, there is a clear correlation between the simulated and measured antenna parameters. However, minor variations are observed due to manufacturing and soldering errors, but they are tolerable.

MIMO parameter analysis of 2 × 2 UWB MIMO antenna
As part of an integrated antenna system, the 2 × 2 UWB MIMO antenna is used for UWB MIMO operations. A 2 × 2 UWB MIMO antenna's MIMO performance is evaluated using MIMO parameters. MIMO parameters are given as a table instead of a plot for the purpose of simplicity and to make it easier to read. As indicated in Table 3, four frequencies (3.5, 6.5, 9, and 10 GHz) are selected for study in the 3-13 GHz range.

Functional verification of CR and UWB MIMO antenna
This section describes the hardware connection, wide band spectrum generation, and detection. The spectral sensing and communication capabilities of the designed integrated CR antenna is evaluated using a fabricated prototype of the antenna, NI USRP 2943R, the bayesian learning algorithm and NI LABVIEW software monitor. Figure 21 depicts a block diagram of the USRP-based 3 GHz (3-6 GHz) spectrum generation, spectrum detection block, white space detection, and communication systems. USRP device with proposed antenna is used for spectrum generation, detection, and communication function verification. The USRP has two channels: RF0 and RF1, and each channel (RF0 and RF1) has two ports. Furthermore, the device can transmit and receive spectrum ranging from 1.2 to 6 GHz. The proposed CR and UWB MIMO functional verification block diagram is shown in Fig. 21.  www.nature.com/scientificreports/ In order to verify the CR antenna system functional verification following procedure is followed • A UWB antenna is connected to an RF0 port to generate a 3 GHz (3-6 GHz) wide spectrum.
• Another UWB antenna is linked to the USRP RF0 Rxr port to detect a 3 GHz (3-6 GHz) spectrum, and four NB communication antennas are linked to USRP RF1.   www.nature.com/scientificreports/ • Then, the receiving UWB antenna detects the generated 3 GHz (3-6 GHz) wide spectrum and feeds it to the USRP RF0 Rx port. • A real-time algorithm is used to visualize the USRP received signal in LABVIEW and it is shown in Fig. 22.
• In addition, GNU Radio and MATLAB software are utilized to execute processing on the PC using an FFTbased algorithm to generate the 3 GHz (3-6 GHz) spectrum as shown in Fig. 23 . • The processed 3 GHz (3-6 GHz) spectrum is shown in the Fig. 23. Further, in order to find the white space in the detected 3 GHz (3-6 GHz) spectrum, we have used Bayesian learning-based spectrum sensing algorithm in MATLAB. • After the process the occupied and free spectrum is identified as depicted in Fig. 24. It is witnessed from the figure there are five white space in this detected spectrum, which is at 3.3, 3.5, 5, and 5.5 GHz.

Conclusion
In this article, the equivalent circuit models have been proposed for NB antennas, UWB antenna, and band notch filters. The lumped element model for the UWB antenna is achieved after many optimizations. The lumped element's value is calculated approximately and optimized in AWR, ADS, and CST to obtain an accurate lumped  www.nature.com/scientificreports/ element (R, L, and C) value. These equivalent circuit models of NB antennas, UWB antenna, and band notch filters show identical performance between the equivalent circuit model and EM model results. The simulated and measured results are analyzed using important antenna parameters. Also, the functional verification of the integrated antenna is carried out using USRP, and it is witnessed that the integrated antenna is suitable for CR spectrum sensing and communication applications and short range video transmission applications.